Polyamide-imides are condensation polymers finding use in a wide range of applications such as adhesives, molding compositions, fibers, films, composites, laminates, etc., owing to a desirable combination of properties. Torlon.RTM. polyamide-imides, available from Amoco Performance Products, Inc., are examples of commercial polyamide-imides.
Despite the many desirable properties of polyamideimides, their utility in certain applications has been limited by a tendency to absorb water, leading to loss of mechanical properties on exposure to high temperatures. So-called annealing or post-curing treatments of polyamide-imide or polyamide-amic acid fabricated parts, as disclosed in commonly assigned U.S. Pat. No. 4,167,620, in which water liberated due to imidization and chain extension reactions as well as absorbed moisture are allowed to diffuse out of fabricated parts with controlled heating, improve retention of mechanical properties. However, such treatments are time-consuming and do not give sufficient improvement of currently available, commercial polyamideimides for certain demanding applications. It also is known to add certain metal oxides to polyamide-imides to tie up absorbed moisture as well as water liberated during imidization and chain extension reactions to avoid cracking and sacrifices in physical properties. Again, however, this approach does not yield sufficient improvement of conventional materials for demanding applications.
Aromatic polyimides typically do not absorb water to as great an extent as polyamide-imides. This may be due to the lower levels of amide bonds in polyimides and the greater affinity of water to amide bonds than imide bonds Of course, it also is well known that utility of aromatic polyimides is limited because of their lack of solubility. Further, their high glass transition temperatures ("Tg") make melt processing impractical or impossible.
U.S. Pat. No. 4,017,459, assigned to the Upjohn Company, discloses amide-imide polymers and copolymers prepared from 2,2-bis(4-(p-aminophenoxy)phenyl)propane and trimellitic anhydride halide or from 2,2-bis(4-(p-isocyanatophenoxy)phenyl)propane and trimellitic acid or anhydride. According to the patent, such polyamide-imides are melt processible, such as by injection molding, and useful in manufacture of articles such as gears, ratchets, clutch linings, bearings, pistons and cams and electrical components. In contrast, the patentee teaches that polyamides prepared from the above-named diamine and isophthalic acid and polyimides prepared from that diamine and pyromellitic acid dianhydride or benzophenone tetracarboxylic acid dianhydride are intractable in the sense of lacking sufficient solubility for solution processing, lacking in melt processibility or lacking both.
U.S Pat. No. 4,111,906 and 4,203,922, both assigned to TRW, Inc., disclose that processibility of polyimides can be improved by using the same in predominately polyamide-amic acid form and imidizing during a final fabrication step but that such an approach is disadvantageous because voids in the final products result from water liberated due to the imidization reaction. These patents also state that chemical and thermal stability are improved by preparing polyimides from 2,2-bis(4-(p-aminophenoxy)phenyl)hexafluoropropane. According to the '906 patent, polyimides prepared from this diamine and a dianhydride are useful as coatings, adhesives and as a matrix for laminated glass or graphite structures. Polyimide foams prepared from pyromellitic acid dianhyride or other aromatic tetracarboxylic acid dianhydrides and such diamine in combination with a second aromatic diamine are disclosed in U.S. Pat. No. 4,535,101, assigned to Imi-Tech Corporation. Preparation of polyamides from the above-named diamine and diacids also is disclosed in the '906 patent. The abstracts of both the '906 and '922 patents mention polyamide-imides; however, no additional information is provided.
U.S. Pat. No. 4,340,697, assigned to Toray Industries, Inc., discloses melt processing difficulties with polyamideimides and purports to remedy the same by blending with polyphenylene sulfide, polyamide, aromatic polyester, polyphenylene ether or a phenoxy resin. According to this patent, polyamide-imides may contain, in addition to a repeating, main structural amide-imide unit, up to 50 mole percent amide or imide units, the latter being introduced into the polymer by replacing aromatic tricarboxylic acid component used in preparation with pyromellitic acid dianhydride or benzophenone tetracarboxylic acid dianhydride. A bis 4-(m-phenoxyphenyl)sulfone structure also is depicted among the multitude of chemical formulas illustrated in this patent. In contrast, the only polyamideimides employed in the examples are identified as Torlon.RTM. polyamide-imides manufactured by Amoco.
U.S. 4,485,140 and 4,576,857, both assigned to E. I. DuPont de Nemours and Company, disclose melt fusible, though not necessarily melt processible, polyimides useful as binders for advanced composite materials. These polyimides are prepared from pyromellitic acid dianhydride or a diester thereof, optionally in combination with minor amounts of another tetracarboxylic acid compound, and various diamines, including bis-(aminophenoxyphenyl)propanes or 2,2-bis(3,5-dichloro-4-(4-aminophenoxyphenyl)propane. Use of an excess of one of the components, preferably the diamine, to limit molecular weight build-up and capping of reactive species with monofunctional reagents, such as phthalic anhydride or phthalic acid or aniline, to enhance stability of melt viscosity also are disclosed. Low-void or void-free composites containing graphite, glass, aramid or other high strength fibers impregnated with such polyimides and preparation thereof by impregnating a fibrous substrate with a solution of polyimide precursors, partially removing solvent and forming the result at elevated temperature as either a single layer or multiple layers also are disclosed.
U.S. Pat. No. 4,599,383, assigned to NTN-Rulon Industries Co., Ltd., discloses compositions having improved water absorption properties containing a polyamide-imide resin in combination with a polyetherimide and a fluoro resin component.
Published European Patent Application No. 86303748.9 (Publication No. 0 163 518), filed in the name of M & T Chemicals, Inc., discloses polyimides, polyamide acids, polyamide-imides, polyesterimides and polyesteramides containing at least 10 mole percent of a reaction product of an aromatic or aliphatic mono- or dianhydride and certain aromatic diamines having an unsubstituted or halogen- or hydrocarbyl-substituted, para-phenylene radical linked by like or different alkylene, alkenylene, sulfide or oxy groups to two unsubstituted or halogen- or hydrocarbyl-substituted, monovalent, aminophenyl radicals provided that the linking groups are not contemporaneously both sulfide or oxy. Such products are said to exhibit improved processing characteristics and thermal stability and to have utility in widespread applications. Two polyamide-imides and films thereof are demonstrated in the examples. Numerous anhydrides, dianhydrides and diamines are named in this publication and mixtures of anhydrides and dianhydrides are mentioned. It is also reported that mixtures of the above-described diamines with other diamines may be used. Interestingly, such other diamines are said to include 2,2-bis(4-(p-aminophenoxy)phenyl)propane and -sulfone, although the publication also mentions, with supporting citations, that polyimides prepared from such diamines and dianhydrides are insoluble and that polyamide-imides prepared from such diamines are of uncertain solubility and processibility.
While the patents and publication discussed above disclose various polyamide-imides and polyimides, the improved polyamide-imide compositions and composites according to the present invention are neither disclosed nor suggested. Further, from the teachings of such patents and publication, it could not have been predicted that the invented compositions would exhibit the processibility, reduced water absorption and other desirable properties achieved according to this invention.
It is an object of this invention to provide improved polyamide-imide compositions, including amic acid precursors thereto, and filled molding compounds, polymer blends and composites based thereon. A further object of the invention is to provide for preparation of such compositions and use thereof in manufacture of fabricated articles. A further object is to provide improved polyamide-imide compositions well suited for use in composite structures. Other objects of the invention will be apparent to those skilled in the art from the following.
We have now found that the objects of this invention can be attained by providing polyamide-imide compositions, including polyamide-amic acid and polyamide-imide-amic acid precursors thereto, comprising recurring amide-imide or amide-amic acid units and recurring imide-imide, imide-amic acid or amic acid-amic acid units containing a substituted or unsubstituted bis-(phenoxyphenyl) group in which the phenoxyphenyl groups are joined directly or by stable, non-hydroscopic linkages. Such compositions contain an excess of imide or imidizable amide linkages relative to more conventional polyamide-imides. However, imide or imidizable units are not present in sufficient excess to render the invented compositions intractable; accordingly, solution and melt processibility are sufficiently great that composite prepregs can readily be prepared by solution-impregnation of fibrous substrates and single or multi-layered, shaped composites can be formed by application of heat and pressure. Melt processibility is such that filled or unfilled compositions according to the invention can be injection molded or otherwise formed into useful articles.